1. Field of the Invention
The present invention relates to a stepless speed changing apparatus used in the transmission system of various automobile auxiliary equipment powered by, for example, an internal combustion engine.
2. Description of Related Art
Recently an automobile is equipped with a number of auxiliary equipment such as hydraulic pumps to supply hydraulic oil to a power steering device and/or an automatic speed changing apparatus, a battery charging generator and a compressor for an air conditioner. Many of these auxiliary equipment are driven by the force generated by the engine and transmitted by a belt via a crank pulley mounted at the end of a crank shaft, for example.
In the case where such auxiliary equipment is driven by directly transmitting the rotational force of the engine, considerable power loss is caused when the engine runs at a high speed. In the case where the specifications of the auxiliary equipment; are determined aiming at the minimization of the power loss, on the other hand, there arises such a problem that the auxiliary equipment cannot operate with sufficient performance when the engine runs at a low speed. Therefore, such a configuration has been employed in the prior art that a speed changing apparatus is interposed at an intermediate point of the transmission system leading to the auxiliary equipment wherein the reduction ratio of the speed changing apparatus is adjusted according to the rotational speed inputted from the engine. The driving force is directly transmitted to the auxiliary equipment when the input rotational speed is low. Speed reduction at a specified reduction ratio is carried out before the rotational speed is transmitted to the auxiliary equipment when the input rotational speed is high, thereby solving the problem described above.
Speed changing apparatuses which can steplessly change the speed have widely been used for the speed changing apparatus. This is for the purpose of preventing a sudden change in the engine load caused by the change of the reduction ratio and deterioration of driving stability. Changing of the speed reduction ratio in the speed changing apparatus is generally carried out by detecting the input rotational speed of the engine and actuating a speed changing actuator according to the detected speed. However, such a configuration results in a complicated and large-sized overall configuration of the speed changing apparatus, and hence it is difficult to secure an installation space in the transmission system and is likely to cause malfunction due to external noise.
The above discussion implies that a preferred configuration for a speed changing apparatus used in the transmission system of automobile auxiliary equipment is one capable of steplessly changing the speed ratio according to the input rotational speed using simple mechanical means. Stepless speed changing apparatuses achieving such an object are disclosed in Japanese Patent Application Laid-Open Nos. 63-82876 (1988), 61-228155 (1986) and U.S. Pat. No. 4738164.
The stepless speed changing apparatus disclosed in Japanese Patent Application Laid-Open No. 63-82876 (1988) was proposed as an application to a hydraulic pump for supplying the hydraulic oil to a power steering apparatus. An input shaft driven by the force from the engine and an output shaft driving the hydraulic pump are connected via a variable pitch pulley which changes the speed reduction ratio by changing the effective diameter thereof. A steel ball is disposed on the outside of a half of a conical body constituting the variable pitch pulley on the output shaft side. The steel ball moves outward in the radial direction under the action of the centrifugal force generated by the rotation of the output shaft, thereby pressing the half body inward, namely pressing the half body to approach another half.
With this configuration, because the effective diameter of the variable pitch pulley on the output shaft side increases as both half bodies come near to each other according to the movement of the steel ball, and the movement of the steel ball is caused by an increase in the rotational speed of the output shaft, speed reduction ratio from the input shaft to the output shaft is changed steplessly as the rotational speed of the output shaft increases.
The stepless speed changing apparatuses disclosed in Japanese Patent Application Laid-Open No. 61-228155 (1986) and U.S. Pat. No. 4738164 are basically stepless speed changing apparatuses of ring cone type, wherein an input disk which receives the input from the engine and an output disk on the output side to the auxiliary equipment are in rolling contact with a plurality of planetary cones intervening in the circumferential direction between both disks. A speed change ring is provided which is in rolling contact with all the planetary cones on the cone generating lines thereof parallel to the center axes of the input disk and the output disk, So that the positions of rolling contact with the planetary cones are changed by the movement of the speed change ring in the axial direction, thereby changing the speed reduction ratio from the input disk to the output disk steplessly. The stepless speed changing apparatuses further comprise a centrifugal drive gear pivoted on the circumference of a specified circle having the center at the axis of the centrifugal drive gear and an annular cam member having an internal gear which meshes with the outer gear of the centrifugal drive gear are disposed, wherein a roller key projecting from the speed change ring is brought into rolling contact with the cam surface of the cam member.
In such a configuration, when the centrifugal drive gear rotates about the pivot axis, the cam member in mesh therewith via the internal gear rotates to press the speed change ring via the roller key and moves in the axial direction thereby to change the speed. The centrifugal drive gear has a weight portion eccentrically located at a point in the circumferential direction with an offset from the pivot axis, and is driven to rotate about the pivot axis, so that the weight portion is located on the outside in the radial direction, by the centrifugal force which acts according to the rotation on the circle whereon it is installed. Thus the reduction ratio from the input side to the output side is changed steplessly as the rotational speed of the output side increases.
In the stepless speed changing apparatus disclosed in Japanese Patent Application Laid-Open No. 63-82876, the effective diameter of the variable pitch pulley on the output shaft side changes according to the movement of the steel ball caused by the effect of the centrifugal force thereby effecting the speed reduction. However, increasing the effective diameter of the variable pitch pulley on the output shaft side requires a force to resist the tensile Force of a transmission belt connecting the variable pitch pulleys on the output shaft and the input shaft. This force cannot be sufficiently provided by the moving force of the steel ball. This means that the speed reduction with the stepless speed changing apparatus disclosed in Japanese Patent Application Laid-Open No. 63-82876 is difficult to achieve and hence impractical.
On the other hand, the stepless speed changing apparatuses disclosed in Japanese Patent Application Laid-Open No. 61-228155 (1986) and U.S. Pat. No. 4738164 do not need a great force to move the speed change ring in the axial direction for changing the speed and carry out the operation described above without any trouble. However, -these apparatuses need precision parts which require high machining accuracy such as the cam member provided with the internal gear and the centrifugal drive gear which meshes with the cam member, and have a drawback that considerable man-hours is required in the machining and assembly of these parts.
In addition, all the stepless speed changing apparatuses of the prior art constituted as described above carry out the speed changing operation in accordance to the change in the rotational speed on the output side, and are incapable of providing such a characteristic that the output rotational speed decreases as the input rotational speed from the engine increases, which is occasionally required in auxiliary equipment of automobiles.
For example, a hydraulic pump to supply hydraulic oil to a power steering device needs almost no steering assisting force when the automobile is running at a high speed due to a smaller resisting force acting on the steered tires from the road surface, and is therefore required to supply a smaller amount of oil than that supplied during low speed driving. However, a stepless speed changing apparatus of the prior art does not decrease the rotational speed of the hydraulic pump on the output side but increases the rotational speed of the hydraulic pump while decreasing the rate of increase, as the engine speed on the input side increases. Thus when the automobile is running at a high speed, the hydraulic pump is driven at a high rotational speed to supply useless hydraulic oil which is not used in assisting the steering operation. It is known that the power consumed by a hydraulic pump is approximately proportional to the cube of the rotational speed, and the loss of engine power caused by supplying the useless hydraulic oil is significant resulting in deterioration in the running performance and in the mileage of the automobile.
To sum up, a hydraulic pump for a power steering device is required to transmit the rotation without reducing speed when the input rotational speed from the engine is low, and to transmit the rotation after reducing the speed sufficiently when the input rotational speed from the engine is high, though the stepless speed changing apparatuses of the prior art are not capable of achieving such a speed changing operation.